Google Map icon (pin) - how to define color?

You can declare the image when you initialize the map:

var overlay;

function initialize() {
  var myLatLng = new google.maps.LatLng(0, 0);
  var myOptions = {
    zoom: 11,
    center: myLatLng,
    mapTypeId: google.maps.MapTypeId.SATELLITE
  };

  var map = new google.maps.Map(document.getElementById("map_canvas"), myOptions);

  var swBound = new google.maps.LatLng(0, 0);
  var neBound = new google.maps.LatLng(0, 0);
  var bounds = new google.maps.LatLngBounds(swBound, neBound);

  // This is where you declare your image...
  var srcImage = 'images/yourimage.png';
  overlay = new USGSOverlay(bounds, srcImage, map);
}

More info here if you need it:

http://code.google.com/apis/maps/documentation/javascript/overlays.html#CustomOverlays